1. Field of the Invention
The present invention relates to a process for the preparation of a highly water absorptive resin and more particularly, to a process for producing a highly water absorbing resin having a high water absorption rate, and an excellent gel strength and fluidity and its manufacture.
2. Description of the Prior Art
Various water absorbing materials such as natural sponge, pulp, paper and the like, are well known in the art. Also various synthetic products made by graft polymerization of materials having a water absorbing group, such as --OH, --NH.sub.2, --COOH, and the like, have been used conventionally. However, such water absorbing natural materials, e.g. a sponge, a pulp, a paper are characterized with physical mechanism for absorbing water so that such materials have defects in that the majority of the absorbed water can be easily squeezed out by the application of external pressure. In recent years, in order to solve these defects, the synthetic products having particular physical and chemical mechanisms have been developed. The majority of such synthetic water absorptive products are crosslinked polyacrylic acid salt, or polymethacrylic acid salt, crosslinked polyarcylic acid-methacrylic acid copolymer, salt, crosslinked starch-acrylonitrile graft copolymer, and cellulose and acrylate-grafted copolymer. Such grafted products are on the market as sanitary napkins, sanitary pads, diapers in the sanitary field, water absorbing containers in the civil and gardening field, and anti-dewdrop agents in the construction field.
The processes for the preparation of these products are shown in U.S. Pat. No. 4,076,663 which discloses a process comprising graft polymerization of starch and acrylic acid, and hydrolyzing the starch-polyacryl-grafted polymer, and U.S. Pat. No. 4,093,776 which discloses a process comprising the polymerization carried out by suspending and dispersing an aqueous solution of crosslinked sodium polyacrylate. However, the products made by these processes have a lower water absorbing property so that these products are not properly utilized as sanitary pads or paper diapers.
In general, a high water absorptive resin requires water absorption rate, water absorptive capacity, and gel strength and they have mutual opposing properties with each other. On the other hand, each performance is sacrificed to some extent. For example, the sanitary pads or paper diapers require a high absorption capacity as well as a high water absorption rate because the liquid from the human body can contact the human skin so that skin diseases such as edema, dermatitis, or congestive skin are formed on the human skin. Furthermore, the water content can be leaked from the water absorptive products. Accordingly, the necessity of good products for desired pads or diapers requires that there are to be several types of water absorptive resins which are known in the art. One such water absorptive resin is shown in U.S. Pat. No. 4,497,930 which discloses a process for producing a water absorbent polymer comprising the steps of subjecting a crosslinked sodium polyacrylate to have a water content of 10 to 40% wt, and adding a crosslinking agent for crosslinking the surface thereof. However, such water absorptive resin has a insufficient water absorptive property and a lower gel strength.